This invention relates generally to seismic exploration of substrata beneath bodies of water and, more particularly, to a marine seismic cable for sensing reflected seismic waves from such substrata.
Marine seismic exploration is often conducted by towing a seismic streamer at a given depth through the ocean or other body of water. The streamer is provided with a plurality of acoustic sensitive transducers, that is hydrophones, disposed at appropriate intervals along the length thereof. Acoustic wave energy is provided in the vicinity of the cable by an air gun or other suitable means; this wave energy travels downwardly through the earth with a portion of it being reflected upwardly at levels where there is a contrast in the velocity propagation characteristics of the stratum. The hydrophones sense the acoustic pressure waves produced in the water by the upwardly traveling seismic reflections and provide electrical signals indicative thereof to suitable processing and recording equipment located on the seismic vessel that is towing the streamer.
The magnitude of the reflected signals is extremely small, thus making it essential to minimize extraneous noise detected by the hydrophones and to maximize the signal-to-noise ratio. One source of such noise is boundary layer or flow noise which is generated by the water flowing past the surface of the cable in a turbulent fashion. It has been found that flow noise is essentially a localized pressure disturbance, which is normal to the surface of the cable, and that flow noise does not propagate in the water in the acoustic sense but rather is convected by the water flowing past the cable.
Prior art cables have employed mounts, such as that disclosed in U.S. Pat. No. 3,781,778, which have a cavity in which the hydrophone is positioned in an elastic material, for example, polysulfide elastomer. A thin sheet of plastic film is positioned over the cavity, and a sheath or jacket of extruded plastic is provided over the outside of the cable and the mount. The flow noise or pressure fluctuations are transmitted through the cable sheath, the thin plastic sheet and elastic material to the hydrophone, thus being detected as undesired noise.
Therefore, it is an object of the present invention to provide a marine seismic cable that reduces the flow noise sensed by a hydrophone mounted in the cable.